Motor vehicles whose drive and brake systems comprise one or more electric machines are becoming ever more important. Here, the electric machine may often also be used as a drive if a battery of the motor vehicle has stored an adequate amount of electrical energy. It is desirable, for example with regard to the range of an electric drive, for the battery or the batteries of the motor vehicle to be charged not only when the vehicle is at a standstill but rather also during the operation of the motor vehicle. If the electric machine is operated as a generator, then it is possible during the braking of the motor vehicle for electrical energy to be produced and stored in the battery through conversion of the kinetic energy or rotational energy of the axle connected to the electric drive or of the wheels fastened to said axle.
The braking torque built up as a result of the recuperation, that is to say the production (recovery) of electrical energy by the generator, is dependent on the speed of the motor vehicle and the state of charge of the battery. To be able to realize adequate braking deceleration in all situations, brake systems for example for hybrid vehicles, which combine an electric drive and a combustion engine, have additional braking means in addition to the electrically regenerative brake (that is to say the generator). Said additional braking means, which are conventionally realized in two brake circuits as prescribed by law, comprise for example electromechanical, hydraulic and/or electrohydraulic friction brakes.
In particular, the interaction of hydraulic friction brakes and a generator, which is desirable owing to lower system costs and moderate system complexity, presents some technical challenges: for efficient recuperation, the maximum possible braking torque of the generator in the respective driving situation should be utilized. Furthermore, a transition between braking of the motor vehicle through the braking deceleration of the generator and braking of the motor vehicle by means of friction braking should be possible without resulting in abrupt changes in braking deceleration or an unfavorable braking force distribution. Furthermore, a situation must be prevented in which the decoupling of the electrically regenerative braking by the generator from the brake pedal results in an abnormal pedal feel for the driver, such that it is difficult for the driver to discern the braking action attained by him.
DE 196 04 134 A1, which is incorporated by reference, proposes a method and a device for controlling the brake system of vehicles with electric drive, in which, in a first range of driver braking demand, the braking torque is imparted almost exclusively by the regenerative braking, wherein the hydraulically actuable friction brake imparts substantially no braking torque. This is realized in that, in said range of braking demand, the pressure medium which is caused to flow into the wheel brake cylinders as a result of the brake pedal actuation by the driver is, through corresponding control, recirculated into an accumulator chamber.
WO 2004/101308 A1, which is incorporated by reference, discloses a brake system and a method for regulating a brake system of a motor vehicle which has an electrically regenerative brake, in particular a generator, and a number of hydraulic friction brakes driven at least one brake pressure generating means by means of a brake medium, the overall deceleration of which vehicle is composed of deceleration components of the friction brakes and of the electrically regenerative brake. To attain the greatest possible level of braking comfort and acceptable brake feel, brake medium is discharged into a low-pressure accumulator during braking with the electrically regenerative brake. Here, complex regulation takes place in which the hydraulic valves are held in a precisely determined partially open state.
DE102008054859A1, which is incorporated by reference, discloses a method for controlling a brake actuation of a hydraulic vehicle brake system of a motor vehicle which has an electric drive motor which, for braking the motor vehicle, can be operated as a generator, wherein the vehicle brake system has a master brake cylinder which can be operated by muscle force and to which a wheel brake is connected. If, during a brake actuation, the electric drive motor is operated as a generator, then through the opening of the valve, a brake fluid volume is conducted into the hydraulic accumulator and a wheel brake pressure in the wheel brake is thereby reduced. A boost factor of the brake force booster is preferably reduced during a brake actuation with generator operation of the electric drive motor.
A disadvantage of the brake systems described previously lies in the fact that, owing to different friction losses in the two pressure accumulators (as a result of which a piston movement occurs in the event of different pressures) and/or in the thrust rod circuit and in the floating circuit of the tandem master cylinder, a situation may arise in which only the pressure accumulator of one of the two brake circuits is filled. This can lead to an uncomfortable transition (“blending”) from substantially electrically regenerative braking to combined braking with generator and friction brake or purely friction braking.
DE102010039816.0, which is incorporated by reference, discloses a method for operating a brake system of a motor vehicle, wherein the brake system comprises an electric generator and a first and a second hydraulic brake circuit with in each case first and second hydraulic service brakes, a tandem master cylinder, a reservoir for brake fluid and a brake pedal. Upon an actuation of the brake pedal by the driver, the available braking force of the generator is determined, and from this an equivalent brake fluid volume is calculated which corresponds to hydraulic braking of the motor vehicle with the same braking force, and the equivalent brake fluid volume from the second brake circuit is conducted into a volume unit connected to the second brake circuit. This ensures a comfortable pedal feel for the driver, but has the disadvantage that the volume unit, which is formed as an additional component, increases the system complexity.